1. Field of the Invention
This invention relates to printing processes and in particular to dot patterns used when preparing halftone images. These patterns include shapes which reduce the perceptibility of certain moire effects and various tone jumps which are often seen.
2. Description of the Prior Art
In most printing processes it is only possible to apply a single tone of each available ink color to the print media. Tone variation is then achieved by breaking up each image into fine dots of varying size on a halftone screen grid. Color variation is normally achieved by superimposing screens of the primary colors cyan, magenta and yellow, plus black for definition. Ideally human vision integrates the dots over a well prepared image into an accurate impression of the original scene. A final image will almost always include a number of compromises between practical limitations and defects in the printing process, and what can actually be perceived by the human eye and brain.
Round dots at 45.degree. to vertical are least perceptible for a given spacing or screen ruling (or dot frequency), and single color images are conventionally printed in this manner. Other dot shapes or combinations of shapes such as square and "elliptical" (diamond shape) are sometimes used, but all have generally straight or outwardly curved edges around the fill length of their perimeters. The dots are usually created in square cells forming rows spaced at between about 30/cm for newsprint and about 60/cm for higher quality images. In light tones the dots remain distinct on a light background provided by the print medium, but merge in darker tones which then appear as light dots on a dark background. The printed and non-printed areas of an image therefore appear to reverse from dots to background and background to dots respectively as tone darkens.
When printing color images undesirable moire effects in the form of large and small scale patterns are often seen due to periodic alignment of the dots as a whole and of their edges. The large scale patterns are typically bands which intersect to form squares on the order of tens or more dots along each side. This effect is largely removed by suitable relative rotation of the color screens such as by cyan 15.degree., magenta 45.degree., yellow 90.degree. and black 75.degree. anticlockwise from horizontal. Complex mathematical procedures are often used to establish suitable angles. Placing the screens without relative rotation or offset can cause color shifts in an image where colors of differing opacity, particularly black and yellow, overlap consistently over a large region. Subtle color shifts may occur in any case due to mis-registration of the screens during their superposition. The small scale patterns of moire effects are typically rosettes on the order of a few dots width, which cause perceptible speckling of otherwise uniformly colored areas. This effect has proved more difficult to remove.
A further problem often arises in printing halftone images, known as dot gain, amid tones where adjacent dots are so sufficiently large as to become linked by imprecision in their reproduction. For example in lithography, which includes offset printing, a greasy ink is confined to printing areas of an image plate by dampening the surrounding non-printing areas with water. Unfortunately surface tension at the ink/water interfaces can cause or enhance bridging between closely spaced printing areas creating sudden tone jumps. Ink absorption on poor quality print paper can also lead to bridging. Round and square dots formed in square cells naturally meet their nearest neighbors at 78% and 50% printing area densities, respectively. Dot gain causes bridging at slightly higher densities creating discontinuities in regions of an intended smoothly varying tone. This effect is also difficult to remove completely.
Preparation of halftone images is largely carried out using computer controlled devices such as scanners and imagesetters. A photograph or other artwork to be reproduced is scanned and the original scene is stored in electronic memory or output directly. The images can be manipulated and/or combined with text before a printing medium such as a film or plate is produced. It is normally only in the final output stages that an image is converted to halftone dot screens. The manipulations are complex software operations which may be varied to suit particular images. Similar software is used in other electronic printing and imagesetting processes such as desktop publishing. Precise control of the dot patterns is necessary in preparing acceptable images, and the computations required for high quality images are often extensive and time consuming. For example, software methods for reducing moire effects are disclosed in U.S. Pat. No. 4,084,183 (congruent screens), U.S. Pat. No. 4,894,726 (quasi periodic screens), EP 370271 (elongated conventional dots), WO 90/10991 (rectilinear screen transposition) and WO 90/06304 (pseudo random variation of dot shapes). Some images are still prepared using conventional photomechanical equipment such as contact screens. Good summaries of known dot patterns, their various problems and moire effects are to be found in Colour Screening Technology; A Tutorial on the Basic Issues, The Seybold Report on Desktop Publishing, Vol. 6, No. 2, October 1991, Seybold Publications, Inc., PA, USA, and Desktop To Press, Issue 9, February 1992, Peter Fink Communications Inc., CA, USA.